Optical fibers are relatively fragile and must be protected during manufacture and installation. A variety of protective measures are therefore provided in cables containing optical fibers. The optical fiber or fibers are typically enclosed in a plastic buffer tube having a bore of a cross-sectional area larger than the cross-sectional area of the fiber or fibers within it. This is referred to as a "loose" configuration. The material of the tube typically has a relatively high temperature coefficient of expansion and a relatively low tensile strength. Frequently, the axial length of the tube is shorter than the linear length of the fibers or ribbons. The tube can move or be flexed a certain degree by external forces or by thermal expansion and contraction, without bending the optical fiber ribbon.
To further resist thermal expansion and contraction, strength members of metal wires, high strength non-metallic rods or fibers, such as glass rods or fibers or aramid in a matrix of resin, can be provided adjacent the tube or tubes containing the optical fibers. See, for example, U.S. Pat. Nos. 5,509,097 and 5,229,851, assigned to the assignee of the present invention.
Strength members have been provided in the outer jacket or sheath to resist pulling, such as pulling which occurs during installation of a cable. Additional layers of materials, such as armoring for crushing and rodent protection, can also be provided. For moisture protection, the tube is typically filled with a water blocking compound which permits the fibers or ribbons to move within the buffer tubes. The water blocking compound may be a gel or grease-like, and non-hygroscopic and/or thixotropic.
Optical fiber cables are available in a variety of configurations. For example, optical fiber cables are available comprising one or more optical fibers, an optical fiber ribbon or an optical fiber bundle loosely contained within a central tube. Optical fiber ribbons are typically preferred where high fiber counts are required, such as feeder and distribution segments of an optical fiber network. They are also used to connect locations separated by long distances, referred to as long haul applications, such as connecting central telephone stations to local networks. Such cables could also be used in cable TV networks or as data links between computers. In U.S. Pat. No. 5,509,097, described above, the central tube loosely contains an optical fiber ribbon.
Optical fiber cables are also available comprising a plurality of tubes, each containing a plurality of optical fibers in a loose configuration and disposed around a central strength member to resist thermal expansion and contraction. Further strength members can also be provided in an outer protective jacket. Such cables are typically used where the ability to splice to different local points is required. For higher fiber count applications, optical fiber ribbons can be disposed in each of the tubes. See, for example, U.S. Pat. No. 5,229,851.
Optical fiber cables have been proposed which include both a central tube containing optical fibers for long haul applications and a plurality of outer tubes containing optical fibers for shorter distance connections. U.S. Pat. No. 4,822,132, to Oestreich, for example, discloses an optical communications cable for use in local cable networks comprising an inner central tube surrounded by a plurality of smaller tubes, each containing fewer optical fibers than the central tube. The outer tubes are stranded about the central tube in an alternating twist or reverse oscillating lay configuration. The outer tubes are accessible for splicing and branching while the central tube can continue through branching locations to cable terminals. No strength member system is provided to resist longitudinal forces, such as the forces due to installation and thermal expansion and contraction.
U.S. Pat. No. 4,230,395 to Dean et al., discloses an optical fiber cable comprising a plurality of optical fibers loosely contained within a plurality of tubes, surrounded by a sheath. A central tube containing optical fibers can also be provided, and is surrounded by the plurality of non-stranded tubes. Reinforcing members extending parallel to the cable axis are embedded in the sheath, in the tube walls, or can be between the plurality of tubes and the sheath but spaced from the central tube. Thus, the reinforcing members do not resist longitudinal expansion and contraction of the central tube.
U.S. Pat. No. 4,078,853 to Kempf et al., discloses an optical fiber cable comprising a plurality of tubes, each loosely containing an optical fiber ribbon, helically stranded around a central tube, also containing an optical fiber ribbon. An outer jacket reinforced with strength members surrounds the tubes.
However, none of the references cited hereinbefore suggests that when outer tubes containing optical fibers are disposed around a central tube containing optical fibers, structural strength members should be placed between the outer tubes and in contact with the central tube or that the outer tubes with the structural strength members therebetween should be wound around the central tube. It is unexpected that such a configuration would provide better protection for the optical fibers contained within the outer tubes and the central tube from thermal expansion and contraction, because the strength members are not substantially rectilinear and parallel with the cable axis, as in the prior art.